Prior-art legged vehicles, especially those adapted for moving over rough or uneven terrain have been proposed. The terms vehicle, walking machine, and robot are to be construed broadly and includes any means of transportation, whether merely of itself or of objects other than itself. As early as 1898, H. G. Wells described a fictional 100 foot tall, three-legged walking machine in his science fiction novella entitled “War of the Worlds”, and it was first drawn by Warwick Globe circa 1898. In the drawing, the three legs are symmetrically positioned in a triangular pattern to form a tripod stance. About that same time Muybridge used stop-motion photography to study legged locomotion in animals (Muybridge 1899) and later humans (Muybridge 1901). His work provided a method for structuring classical quadruped and biped walking gaits in biologically-inspired legged machines.
Further developments in legged locomotion occurred in the 1960's when research progressed from observation to modeling. It was believed that legged locomotion would increase the speed of vehicles traversing unimproved or rough terrain by a factor of 10×. That is, animals were observed traversing rough terrain at 35 mph while wheeled vehicles managed only 3-5 mph. Additionally, legged locomotion promised better isolation from terrain irregularities. Researchers have investigated four-bar linkages, cam linkages, pantograph mechanisms, and so on, and have built a walking machine with four rectangular frames, controlled by a set of double-rocker linkages, using non-circular gears to produce uniform walking velocity. Additionally, a hexapod and an eight-legged walking machine were developed for lunar rover application. Both walking machines were controlled using mechanical, cam-linkage mechanisms. These designs employed statically stable, symmetric walking gaits, and required moving pairs of opposing legs to keep the body in static equilibrium at all times. These gaits have also been modeled mathematically and diagrammatically, wherein fundamental terminology was defined, such as stance, swing, stride length, duty factor, phase, stability, and so on. For example, a leg is either on the ground, called the stance state or phase, or in the air, called the swing or flight state or phase, and a stride measures the distance the body moves in one stance-to-swing locomotion cycle. The aforementioned prior art vehicles are generally very large, bulky, and cumbersome, and such prior art legged vehicles generally move slower than comparable wheeled vehicles. This highly limits their usefulness.
Further developments in legged walking machines used a computer to control the motion of an eight degrees-of-freedom (DOF) quadruped. The quadruped had two degrees-of-freedom (DOF) for each leg, one DOF at the hip and one DOF at the knee, with independent electromechanical actuators at each leg joint. Using the computer to coordinate or orchestrate the leg joint movements, it demonstrated the classical quadruped walk and trot gaits. Also about that time, General Electric Corporation built a 3000 pound, hydraulically-actuated quadruped that had three DOF per leg, two DOF at the hip and one DOF at the knee. Their quadruped was controlled by a human operator, and it demonstrated that legged machines can move effectively on rough terrain and climb over obstacles, with a human providing control and sensing. Such efforts led to the development of various theories and algorithms for coordinating leg movements in bipeds, quadrupeds, hexapods, and other symmetric legged walking machines to walk over rough terrain, evaluate footholds, and walk outdoors on various types of terrain. In 1968, researchers at Ohio State University proved mathematically that there is an optimal gait for a quadruped that maximizes the longitudinal stability margin. They built a 300 pound hexapod that used force sensors, gyroscopes, proximity sensors, and a camera system to study control algorithms for legged walking machines. Finally, various experiments have been performed on quadruped robots to study walking gaits when one leg is inoperative. In such work, two legs are in-line and the third leg is offset with one of the in-line legs, as in a right-angle triangle orientation, and the two offset legs walk in a predominately bipedal gait with the single in-line leg implementing a hopping motion.
Further developments in legged machines were made to investigate unstable or dynamic legged machines by studying balance of one, two, and four-leg hopping machines. Legged vehicles with less than six legs generally require some degree of dynamic balance to stabilize the body against roll and pitch. The danger of overturning is increased when the legged vehicle is carrying at least one rider or passenger because the rider may make moves which can upset the control system, destroying the normal lateral balance of the legged vehicle and thus causing overturning of the legged vehicle. Prior art stabilization techniques, for example, involve the development of a factor of safety with regard to keeping the center of gravity within the center of pressure of the legs, using a large passive gyro and its precessional momentum to control body pitch and roll, or using retractable outrigger wheels to catch the fall. Such prior art are unsuitable both in terms of weight and use in rugged and uneven terrain. A one-legged hopping machine was investigated. The leg hopping machine was statically unstable and would fall down without constant placement-thrust movement of the foot to compensate for instability. The one-legged hopping machine was modeled as an inverted pendulum and decomposed control into three separate elements: 1) supporting the body by controlling the vertical hopping height, 2) positioning the feet in key locations on each step using symmetry principles to keep the robots balanced, and 3) controlling the body attitude by controlling hip torque during the stance phase such that the dynamic momentum state of the body is estimated ahead in time to calculate the future foot placement and thrust needed to develop complementary dynamic momentum and achieve a desired hopping height, running velocity, and body attitude. This seminal control system demonstrated dynamic re-stabilization against overturning when subject to unexpected forces that destroy the normal lateral balance of the vehicle, thus cause overturning, or when moving on unstable or slippery surfaces, the latter conditions causing foot slip to occur and thus cause overturning. The concept of a virtual leg was developed for modeling dynamic gaits, such as the trot and the pace gaits, whereby symmetric multi-leg machines are modeled as one-leg hopping machines. In other words, for dynamic gaits a biped is modeled as a one-leg machine that alternates the use of left and right legs for support, a quadruped is an extension of a biped when pairs of legs (diagonal pairs for the trot and lateral pairs for the pace) move together and can be modeled as a single virtual leg, and so on. However, this research did not include the bounding gait, where front and rear pairs of legs are moved together, in the same class as the aforementioned trot and pace gaits because it requires using a multi-segmented body to position the virtual foot under the center of mass to provide support, given a body length greater than the leg reach. About the same time, others realized a stable bounding gait for quadruped robots by controlling hip torque during the stance period using a quasi-static slip control algorithm. It was also shown that a simplified control rule stabilizes running without velocity and trunk angle feedback.
Further developments in legged machines have come about because of advances in high-accuracy, high-pressure servo hydraulics combined with real-time low-level control systems. Such legged actuator systems servo positions and forces at the actuated joints to regulate ground reaction forces, maintain support, position, and traction. For example, Boston Dynamics Company built and demonstrated the BigDog quadruped robot. The BigDog quadruped robot has multi-jointed legs adapted for limited oscillatory movement and exhibits a variety of locomotion behaviors: stand up, squat down, walk with a crawling gait that lifts just one leg at a time, walk with a trotting gait that lifts diagonal legs in pairs, trot with a running gait that includes a flight phase, and bound in a special gallop gait. For example, BigDog walks with a dynamically balanced trot gait. It balances using an estimate of its lateral velocity and acceleration, determined from the sensed behavior of the legs during stance combined with the inertial sensors. A high-level control system coordinates behaviors of the legs to regulate the velocity, attitude, and altitude of the body during locomotion. For example, the BigDog control system coordinates the kinematic and ground reaction forces of the robot while responding to basic postural commands. Load is distributed over the stance legs to optimize the load-carrying ability. The vertical loading across limbs is kept as equal as possible while individual legs are encouraged to generate ground reactions towards the hips, thus lowering required joint torques and actuator efforts. A gait coordination algorithm, responsible for inter-leg communication, initiates leg stance transitions to produce a stable gait. A virtual leg model coordinates the legs. The control system adapts to terrain changes through terrain sensing and posture control.
Further developments in legged machines have been realized by improvements in low-power, high computational throughput, self-contained computer systems capable of receiving sensory input, calculating the system and leg kinematics, and controlling each leg joint. For example, a novel tripod robot was designed with omnidirectional legs and body such that the body rotates in the pitch and yaw axis allowing a leg to swing under the body to afford pairs of legs to contact the ground simultaneously. For example, a machine vision algorithm developed by this inventor uses visual data to find the gravity vector in man-made environments, a form of dead reckoning used for balance.